Cardiac rhythm monitoring device

ABSTRACT

The present invention is a cardiac rhythm-monitoring device, which allows patients to perform a preliminary screening for supraventricular arrhythmia. The device detects beat-to-beat heart rhythms (i.e. the R-R interval between individual heart beats) and performs a screening test to determine if there are indications of arrhythmia. The test looks for variance in the R-R interval that is outside of the normal range, either using a pre-constructed chart based upon general population studies to determine the normal range of variance or using normal distribution analysis of the patient&#39;s own heart rhythm to determine the normal range of variance for determining irregular heartbeats, and if there are multiple irregularities within the sensed time frame, the patient is warned of potential supraventricular arrhythmia. By sensing both electrical impulses form the heart and mechanical responses to the heartbeat, the device may augment its analysis.

This application is a continuation of application Ser. No. 10/156978,filed on May 29, 2002.

BACKGROUND OF THE INVENTION

Supraventricular arrhythmia is a particular type of electricaldisturbance of the rhythm of the heart. It is an arrhythmia originatingin the upper chambers of the heart, which can lead to an irregular,abnormal heartbeat. Supraventricular arrhythmia is a fairly commonoccurrence, and as people get older, their chance of experiencingsupraventricular arrhythmia will typically increase. Supraventriculararrhythmia itself is not an immediately life-threatening condition.Current research, however, has revealed that supraventricular arrhythmiapredisposes a patient to such life threatening conditions as stroke,cardiomyopathy, and congestive heart failure. Consequently, it isimportant for doctors to be able to detect supraventricular arrhythmiaas early as possible, since the earlier that supraventricular arrhythmiais diagnosed and treated, the greater the chance that the arrhythmia orits dangerous side effects can be treated, reducing the risks of stroke,cardiomyopathy, and congestive heart failure. Furthermore, it isimportant to be able to monitor the arrhythmia over time, since durationis an important factor in evaluating the health risks associated withsupraventricular arrhythmia and ongoing monitoring also allows doctorsto regulate the amount of medication that patients take as treatment forsupraventricular arrhythmia.

Unfortunately, patients often will not even realize that they areexperiencing supraventricular arrhythmia, since secondary symptoms maynot appear or may be difficult to recognize. The standard technique fordetecting arrhythmias employs an electrocardiogram (“ECG”), which usesseveral electrical leads attached to the patient's chest to monitor thepatient's heart during a visit to the doctor's office (where the ECGmachine is located). An ECG is a medical diagnostic device that producesa fairly detailed readout of the patient's heart rhythm, which a medicalprofessional may interpret in order to evaluate how a patient's heart isfunctioning during the visit to the doctor's office. An ECG often doesnot monitor a patient's real-time beat-to-beat rhythm, however; instead,it may process the patient's heart rhythm over discrete time intervals(such as 5 seconds) to provide a “snapshot” heart rhythm output forinterpretation by a medical expert. Consequently, it may be difficultfor even medical professionals to detect supraventricular arrhythmiausing an ECG. A real-time, beat-by-beat analysis of the patient's heartrhythm would allow for more accurate assessment of a patient's heartrhythm in order to detect the presence of an arrhythmia.

Furthermore, supraventricular arrhythmia is often an intermittent,sporadic condition, such that the use of an ECG during a visit with adoctor may not reveal any irregularity in the heart rhythm, since thepatient may not be experiencing the arrhythmia at that time. In such acase, regular (periodic) or continuous monitoring of the patient's heartrhythm would be better able to detect supraventricular arrhythmia. Oncesupraventricular arrhythmia has been detected, regular monitoring isalso recommended in order to determine the duration of the arrhythmia,since sustained supraventricular arrhythmia lasting more than 24 to 48hours greatly increases the chance of a blood clot forming that couldcause a stroke in the patient.

Regular monitoring would also allow for adjustment of the dosage ofmedications treating the supraventricular arrhythmia (or the secondarysymptoms) based upon the patient's daily condition. Once an arrhythmiahas been detected, doctors often prescribe blood-thinning drugs(anti-coagulants), such as Coumadin® (Warfarin Sodium), in order toreduce the chances of blood clot formation, or anti-arrhythmiamedications, in order to stabilize the heart rate. Unfortunately, boththe blood-thinning drugs and the anti-arrhythmia medications may haveside effects, some of which can be medically serious. Therefore, doctorsmay prefer daily monitoring of the patient's heart rhythm forsupraventricular arrhythmia, so that they may lower the dosage of drugsthat the patient takes as the arrhythmia subsides (as opposed to thecurrent practice of maintaining the same dosage level between doctorvisits, which are typically spaced six months apart). This may reducethe side effects experienced by the patient. A device that a patientcould use to monitor their heart rhythm, searching for signs ofsupraventricular arrhythmia, would address all of these needs. Since apatient would operate such a device, it should be simple, portable,convenient, low-cost, self-contained, non-invasive, and automaticallyassess the patient's likelihood of arrhythmia.

The present invention of the Cardiac Rhythm Monitoring Device (“CRMD”)is designed to perform all of these functions. It is not designed to beused exclusively by doctors as the primary device for sensing,detecting, diagnosing, or classifying arrhythmias. Rather, the CRMDallows for periodic monitoring of the rhythm of a patient's heart,warning the patient if it detects potential supraventricular arrhythmiaactivity. This can be useful when a doctor suspects that a patient hasexperienced intermittent supraventricular arrhythmia, but the ECG doesnot record any abnormal heart rhythms during the visit to the doctor'soffice. The CRMD can provide a preliminary warning of the possibility ofa serious supraventricular arrhythmia, alerting the patient to see adoctor for a more thorough analysis of their rhythms. If the CRMDdetects supraventricular arrhythmia for over a 24-hour period, forexample, the patient has a greater need for medical attention than ifthe duration of the arrhythmia is shorter. And, the CRMD can be used inconjunction with blood-thinning medications or anti-arrhythmiamedications to regulate the dosage according to the condition of thepatient's heart rhythm.

The CRMD may be used in a discrete, periodic manner, or it may be usedto continuously monitor the patient's heart rhythm. In the firstembodiment, the patient, as described above, would typically use theCRMD periodically. In the second embodiment, however, the CRMD couldalso be used continuously, which would be especially useful fordetecting intermittent supraventricular arrhythmia. In that case, thepatient would wear the CRMD continuously throughout the day. This wouldallow for continuous, uninterrupted monitoring of the patient's heartrhythm, such that the CRMD would be able to warn the patient immediatelywhenever it detects a potential supraventricular arrhythmia. The patientwould then be able to seek prompt medical treatment from medicalprofessionals, who could apply confirmatory tests to verify anarrhythmia and provide the appropriate level of treatment to thepatient. For this type of continuous monitoring to be effective,however, the CRMD must not interfere substantially with patient'slifestyle (or else the patient will not wear it). Thus, conveniencefactors (such as small size, light-weight, and unobtrusiveconfiguration) will be incorporated into the design of the device.

SUMMARY OF THE INVENTION

The Cardiac Rhythm Monitoring Device (“CRMD”) is a simple, user-friendlymedical device designed to be operated by a patient, without the needfor extensive training. It monitors the patient's heart rhythm anddetermines if a patient is likely experiencing supraventriculararrhythmia. The CRMD may be used periodically, for regular checks of apatient's heart rhythm, or it may be used in a continuous, uninterruptedmanner, for constant monitoring of a patient's heart rhythms. When usedby a patient under the guidance and supervision of medicalprofessionals, the CRMD can aid in the detection of intermittentsupraventricular arrhythmia, can assist in determining the duration ofthe arrhythmia, and can assist in customizing the appropriate dosage ofmedication to fit the patient's specific needs.

To effectively detect supraventricular arrhythmia, an analysis of thepatient's beat-to-beat heart rate must be performed. The CRMD monitors apatient's beat-to-beat heart rate over a period of time, and analyzesthe patient's heart rhythm to determine if it indicates that the patientis experiencing supraventricular arrhythmia. The CRMD is not designed toperform the extensive and detailed tests which would be required inorder to develop a final diagnosis concerning the patient's heart rhythm(that would, for example, specifically classify the type of arrhythmiabeing experienced). Instead, the CRMD is designed to primarily serve apreliminary screening function. Thus, if the CRMD indicates that apatient may be experiencing supraventricular arrhythmia, the patient isdirected to seek medical attention so that medical professionals mayperform more extensive tests to diagnosis the specific problem and todevelop an appropriate treatment regimen.

The CRMD will often be used only periodically. For example, the CRMD maybe used approximately once per day by the patient in order to check ifthe supraventricular arrhythmia lasts more than 24 hours. Arrhythmia ismore dangerous if it lasts more than 24 hours, since prolongedarrhythmia encourages blood clotting which could, in turn, lead tostroke or other complications. Thus, if repeated, periodic uses of theCRMD indicated that the patient's arrhythmia has lasted more than 24hours, the patient may need to seek immediate medical attention.

Furthermore, once a patient has been diagnosed with supraventriculararrhythmia, the doctor will often prescribe blood-thinning medication,such as Coumadin® (Warfarin Sodium), in order to reduce the likelihoodof blood clot formation, or anti-arrhythmia medications, in order torestore the patient's heart rate to a more normal rhythm. Unfortunately,these medications may have fairly dangerous side effects themselves,which could become life-threatening. For example, patients onblood-thinning medications are at an increased risk of bleeding.Therefore, it is preferable to use these blood-thinning medications onlyfor as long as necessary (i.e. during supraventricular arrhythmiaactivity) and only in the lowest effective dosages, in order to limitthe risk of bleeding to the patient. The CRMD may be used to monitor thepatient's heart rhythm on a daily basis in order to allow for medicalpersonnel to determine the appropriate dosage of medication to be usedby the patient while the supraventricular arrhythmia continues, and todetermine when the arrhythmia has ended and the drugs are no longerrequired.

The CRMD device is essentially comprised of one or more sensors, whichdetect the R-R interval signal of the patient's heart rhythm (i.e. theactual beat-to-beat heart rate); a memory storage means, such as one ormore computerized arrays, which stores the sensed beat-to-beat heartrhythm over a period of time in order to allow for proper analysis todetermine if a warning regarding arrhythmia is warranted; and aprocessing unit, which executes an algorithm to analyze the sensed heartrhythm searching for indications of arrhythmia.

The primary input (from the sensors) to the processing unit consists ofthe measured R-R interval of the patient's heart rhythm. This is abeat-to-beat input, indicating the amount of time between eachindividual heartbeat. Most commonly, the CRMD senses the R-R intervalusing two conductive electrodes, one contacting on the left side of thepatient's body and one contacting on the right side of the patient'sbody, in order to sense the electrical current flow through thepatient's heart. This is a non-invasive means for sensing the patient'sbeat-to-beat heart rhythm, making the CRMD user-friendly. When thepatient's heart beats, the electrical impulse of the heart istransmitted throughout the patient's body. It is this very weakelectrical signal, emanating from the patient's heart and indicatingpotential contraction of the heart as a beat, which becomes the primaryinput signal. The two electrodes of the CRMD receive this weakelectrical signal from the patient's body. The signal is then typicallyamplified, digitized, and normalized before being transmitted to theprocessing unit for analysis. Although other sensing mechanisms (such aspulse oximeters, thermistors, optical electrodes, peak blood flow(pulse) and pressure sensors, capacitance/induction sensors,infrared/photoelectric sensors, and impedance sensors) could be used tomeasure the R-R interval, conductive electrode sensors provide aneffective combination of ease-of-use, convenience, cost-effectiveness,and accuracy.

The processing unit uses the input data from the sensors to determine ifthe patient is likely experiencing supraventricular arrhythmia byanalyzing the regularity of the patient's heart rhythm. A heart rhythmwill have some variation, even in a perfectly healthy, normal person.For a normal, healthy person, however, the variance is limited. So forinstance, general population studies have indicated that the typicalvariance in time between heart beats for a healthy, normal heart is lessthan 125 milliseconds beat-to-beat. Extrapolating from this evidence, itstands to reason that if the signal from a patient indicates a varianceoutside of the normal range (i.e. more than a variance of 125milliseconds between heart beats based on the example study), then thereis a potential problem that could indicate arrhythmia. Morespecifically, the analysis performed by the processing unit wouldcompare each R-R interval between heart beats to the average R-Rinterval for the patient. For a normal, healthy heart, the difference(i.e. variance) between each particular R-R interval and the calculatedaverage R-R interval is less than or equal to 125 milliseconds; anyvariance greater than 125 milliseconds (above or below the average R-Rinterval calculated for the patient) would be indicative of a potentialproblem. Each such event is a potential irregular heart beat outside ofthe normal R-R interval variance range (based on general populationstudies). This type of analysis essentially uses a pre-constructed datatable based on general population studies to determine the appropriatecriteria for classifying individual heart beat intervals as irregular.Obviously, the precise criteria could vary depending upon the study usedas the basis for comparison (and a variance of 125 milliseconds ismerely an example).

Alternatively, the processing unit could analyze the patient's sensordata for irregular heart beats using a statistical approach based uponnormal distribution analysis of the patient's own heart rhythm. Again,the processor would be searching for irregular heart beats which areoutside of the normal range of variance typically seen in human hearts,but this form of analysis effectively builds a data table to determinethe appropriate criteria for classifying individual heart beat intervalsas irregular instantaneously, using the patient's own individual heartrhythm to shape the criteria rather than basing the criteria on moregeneral factors from studies of broader populations. Thus, this approachhas the benefit of being customized to the particular patient. In thisstatistical approach, each sensed beat-to-beat heart beat interval isnormalized using a standard normalization technique (such as dividingeach interval by the average interval, for example). This allows normaldistribution pattern analysis techniques to be utilized to identifyheart beat intervals which are irregular and to indicate an unstableheart rhythm. Only normalized heart beat intervals at the extreme rangeof the normal distribution curve indicate irregularity, so for instance,those normalized values falling outside of the range of 0.95 to 1.05might be classified as irregular.

It is, however, fairly common for even healthy persons to experience anoccasional variance in the interval between heartbeats which is slightlyoutside of the normal range. In other words, occasionally even a normal,healthy heart will produce an extra beat or skip a beat. One or morebrief events outside a patient's normalized R-R interval variance rangemay not indicate supraventricular arrhythmia. Consequently, the CRMDwill analyze the patient's beat-to-beat heart rhythm over some period oftime, 10-60 seconds for example, or over a certain number of heartbeats, with a sample data set typically ranging from 8 to 32 heartbeats, to determine if there is a sustained irregularity that mayindicate supraventricular arrhythmia. If the CRMD detects severalirregular heartbeats with varying R-R intervals in the appropriate timeframe (for example 3 or more within a minute) or a statisticallysignificant number of normalized heartbeats indicating irregularitywithin a sensed data set, then that may be a strong indication ofsupraventricular arrhythmia. If the CRMD is used for an even longerperiod of time (greater than the base period), its analysis shouldbecome even more accurate, since the calculated R-R interval baselinewill become more accurate as additional heart beats are factored intothe calculations, allowing the CRMD software to more effectivelyidentify irregular heart beats.

In order to further improve the analysis performed by the CRMD,additional inputs related to the patient's beat-to-beat heart rate mightoptionally be used in addition to the primary sensor input. For example,the CRMD may optionally sense for peak blood flow, using a pulsepressure sensor, and/or for pulse oxygenation, using an optical sensorto monitor changes in the oxygenation of the patient's blood, in orderto compare these mechanical/physiological indications of the patient'sheart rhythm to the electrical R-R interval indication of the patient'sheart rhythm. Such optional secondary inputs would serve as a check,verifying the accuracy of the electrical R-R interval input data andscreening out false data (such as electromagnet noise from theenvironment around the patient). The secondary inputs would also providemore detailed information about the patient's heart rhythm for analysis,since the amount of lag time between the electrical signal (generated bythe heart and sensed by the electrodes of the CRMD) and themechanical/physiological (blood flow) responses also changes when apatient is experiencing supraventricular arrhythmia. The measured delaybetween the electrical signals and the mechanical signals can alsoassist in the detection of a patient experiencing arrhythmia.

In addition to the required elements of one or more sensors fordetecting the patient's beat-to-beat heart rhythm, a temporary memoryblock to store heart beat (R-R interval) data for the required durationneeded to analyze the patient's heart rhythm, and a processing unit toanalyze the data to determine if there are indications ofsupraventricular arrhythmia, there are also some additional features forthe CRMD. First, the CRMD will require a power source to operate. Whileit could be plugged into a wall outlet to run off of centralized power,an independent, portable power source, such as one or more batteries,would be more convenient. A casing or housing, which could protect theprocessing unit from potentially damaging events and could protect thepatient from electrical shock, would also make the CRMD a more durableand safe device.

In addition, an output unit may be provided so that the patient mayreceive indication of the test results from the CRMD (although the CRMDmay be designed so that it plugs into an external output device aswell). For convenience, the output unit may be integrated into the CRMDdevice itself. For example, the output unit may indicate a warning byilluminating a red light when a problem is indicated. Or, a moresophisticated LCD-type screen could provide the patient with moredetailed information concerning the test results and the recommendedcourse of action. The output device need not be visual either. It could,for example, provide an audible alarm, or it could utilize a voicesynthesizer to communicate with the patient. Further, the full outputunit does not need to be incorporated within the CRMD. Instead, the CRMDcould transmit its results to another, separate device for output to thepatient or medical professionals. For example, the patient coulddownload the results onto their personal computer, which would displaythe output in a form that the patient could understand. Or, the resultsfrom the processing unit could be transmitted over phone lines or overthe Internet to the doctor's office, so that medical professionals mayreview the output results and discuss the information with the patient.

Another optional element, which may be added to the CRMD, would beadditional non-volatile memory storage space. Then, for example, theCRMD (used in a continuous monitoring format) could log a patient'sheart rhythm over a longer period of time; say 24 hours, so that adoctor could review the actual beat-to-beat heart rhythm of the patientwhile diagnosing the patient's condition. Or, the CRMD (used in aperiodic manner) could record the time and date of each test by thepatient along with the results, so that a doctor could chart a patient'sstatus over a longer period of time, in between monthly visits forexample. Thus, the CRMD could be used as an additional source ofinformation about the patient's heart rhythm as medical professionalsdiagnosis the patient's condition, develop an appropriate treatmentregimen, and monitor the patient's progress under the treatment regimen.

In the most typical arrangement of the CRMD configured for periodictesting, the CRMD would be housed in a casing with integrated handles.Each of the handles would have a conductive electrode to sense thepatient's beat-to-beat (R-R interval) heart rhythm. Optionally, thisversion of the CRMD could also have secondary sensors incorporatedwithin it. For example, besides the conductive electrodes in thehandles, which directly sense the heart's electrical impulses, thehandles could also have a finger pulse oximeter incorporated to measurethe patient's mechanical/physiological (blood flow) responses. Thesensor data would be transmitted to the processing unit, typicallylocated in the housing between the two handles, and the results from theprocessing unit would be displayed on a LCD-type screen located atop thecentral portion of the housing, in between the two handles. Thus, thepatient would grip the handles for a period of time, typically 10-60seconds, and would then receive an indication concerning their heartrhythm.

Alternatively, the CRMD could be configured for continuous usage by thepatient. In the most typical arrangement of the CRMD configured forcontinuous monitoring, the patient would wear two sensor bands, on theirwrists, for example. The sensor bands would contain the conductiveelectrodes for detecting the heart's electrical impulses, and could alsoincorporate a pulse pressure-sensing device to monitor the secondary,mechanical/physiological responses of the patient. This data would thenbe transmitted to the processing unit, which would typically be locatedin a housing worn on a belt in the manner of a Walkman™, for example.The housing would also usually include some sort of output device, suchas a warning alarm and/or warning lights, to notify the patient when aproblem has been detected. An LCD-type screen could also be included, toprovide additional details to the patient after the initial warning.Furthermore, one of the sensor bands may also include the housing forthe processor and other microelectronics, including an integrated outputdisplay device.

The object of this invention is to provide either periodic or continuousmonitoring of a patient's beat-to-beat heart rhythm. It is anotherobject of this invention to provide preliminary detection ofsupraventricular arrhythmia. It is still another object of thisinvention to warn a patient of potential supraventricular arrhythmia. Itis yet another object of this invention to allow for detection ofintermittent supraventricular arrhythmia. It is yet another object ofthis invention to screen out noise and singular events which do notindicate supraventricular arrhythmia. It is yet another object of thisinvention to store data concerning the patient's heart rhythm. It is yetanother object of this invention to provide information concerning thepatient's heart rhythm and/or to suggest a course of action to thepatient. It is yet another object of this invention to detect theduration of a supraventricular arrhythmia event. It is yet anotherobject of this invention to allow for a patient's medication dosage tobe regulated and adjusted based upon their detected heart rhythm. It isyet another object of this invention to be sufficiently simple, lowcost, and durable so that patients may monitor their own heart rhythmfor potential supraventricular arrhythmia. It is yet another object ofthis invention to be convenient and unobtrusive so that the CRMD may beworn continuously by a patient without interfering unduly with thecourse of their daily activities.

These and other objects and uses will be apparent to persons skilled inthe art field. Further, a person skilled in the art field willappreciate that there are several different sensing devices, processingunits, memory storage units, output units, power supply sources, casehousing, configurations, and methods of analyzing the patient's heartrhythm for possible arrhythmia which would function in the presentinvention. While several examples are described herein, the presentinvention is not limited to these examples, which are provided merelyfor illustrative purposes. Rather, the present invention includes thesespecific examples and any equivalents. And, although the presentinvention is described primarily as a tool for patients to monitor theirown heart rhythm for signs of arrhythmia, it is to be understood thatthe device is not limited to such use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of the basic steps performed by the CRMD.

FIG. 2 is a flowchart of the analysis algorithm for detectingsupraventricular arrhythmia using only one sensor input regarding thepatient's beat-to-beat rhythm.

FIG. 3 is a flowchart of the more complex analysis algorithm fordetecting supraventricular arrhythmia using secondary sensor inputregarding the patient's mechanical/physiological responses in additionto the primary sensor input regarding the patient's electricalresponses.

FIG. 4.1 is a flowchart of Phase One of the analysis algorithm of thepreferred embodiment, wherein the patient's heartbeat data is normalizedby dividing by subsequent heartbeat data and then analyzed forstability.

FIG. 4.2 is a flowchart of Phase Two of the analysis algorithm of thepreferred embodiment, wherein the patient's heartbeat data is normalizedby dividing by the average heartbeat and then analyzed for stability.

FIG. 4.3 is a flowchart of the base algorithm for detectingsupraventricular arrhythmia with the R-R interval stability indexindicating a triple phase logical agreement of the representative R-Rinterval sample synchronization.

FIG. 4.4 is a flowchart of the base algorithm for detectingsupraventricular arrhythmia with the R-R interval stability indexindicating a sample phase mismatch.

FIG. 5.1 is a graphical illustration of one embodiment of the CRMDconfigured for periodic use.

FIG. 5.2 is a graphical illustration of another view of the CRMDembodiment configured for periodic use.

FIG. 6 is a graphical illustration of one embodiment of the CRMDconfigured for continuous monitoring of a patient's heart rhythm.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The CRMD, generally designated by the numeral 10, is a medicalinstrument that executes a particular procedure to determine if apatient's heart rhythm indicates possible supraventricular arrhythmia.More specifically, in the preferred embodiment the CRMD 10 executes theprocedure shown generally in FIG. 1 for determining if a patient's heartrhythm has indications of supraventricular arrhythmia. First, the CRMD10 senses the patient's beat-to-beat heart rhythm, measuring the R-Rinterval between heartbeats. This data is temporarily stored (typicallyin a computerized array) for use in the detection algorithm. The inputdata or the results may also be stored for analysis later by medicalprofessionals. One of the analysis algorithms shown generally in FIGS. 2or 3, depending upon the number of sensor inputs available, is used todetermine if the patient should be warned about supraventriculararrhythmia. If there is only one sensor input regarding the patient'sbeat-to-beat heart rhythm, then the algorithm shown in FIG. 2 is used;if there are two or more sensor inputs, however, then the more complexalgorithm of FIG. 3 is used to determine whether to warn the patient ofthe potential for arrhythmia. The output data from the algorithm is thendisplayed, either warning the patient of potential arrhythmia orsignaling a normal heart rhythm.

As is discussed above, there are primarily two different physicalvariants of the CRMD 10. The first variant of the CRMD 10 is usedperiodically or episodically to determine if a patient is experiencingsupraventricular arrhythmia at specific points in time. Although thisvariant of the CRMD 10 may be configured in many different ways, FIGS.5.1 and 5.2 illustrate the preferred embodiment. The CRMD 10 shown inFIGS. 5.1 and 5.2 has two handles 20, which are to be grasped by thepatient, one in each hand. Located on the surface of the handles 20 areone or more sensors used to detect the patient's beat-to-beat heartrhythm (i.e. the R-R interval between heart beats). In the preferredembodiment, each handle 20 has a conductive electrode sensor 23 todetect the electrical impulses generated by the patient's heart. Sensorsthat detect heartbeats by sensing the electrical impulse generated bythe heart are typically more accurate and precise, so they arepreferred. Other types of sensors, which detect the body's mechanicalresponse to the heartbeat (such as blood flow changes), could also beused, so long as they provide accurate results.

The sensor does not have to detect the patient's beat-to-beat rhythm atany particular location. Any appropriate location is acceptable,depending upon the configuration of the CRMD 10. In the preferredembodiment, the sensors are located on the handles 20 for ease of use.Optional supplemental sensors 24 could also be included to provideadditional data. For example, when the primary sensor detects electricalimpulses representing each heart beat, the secondary sensor willtypically detect corresponding mechanical/physiological reactions keyedto the heartbeat. In that case, the secondary sensor could be a fingerpulse oximeter, which could be located on the underside of one of thehandles 20 and which would detect the changes in the blood flow causedby the beating of the patient's heart. Also, peak blood flow (pulse)pressure sensors could be located on the other handle 20, to detectpressure changes at one of the patient's pulse points cause by thebeating of the patient's heart. Any sensor, which detects mechanicalresponses, would be appropriate, so long as it is accurate. In thepreferred embodiment shown in FIG. 5.1, the primary sensor is a pair ofconductive electrodes 23, while two secondary supplemental sensors 24are used, a pulse oximeter and a peak blood flow (pulse) pressuresensor.

The beat-to-beat signal detected by the one or more sensors is oftenamplified and normalized in this preferred embodiment before beingtransmitted to a temporary memory storage unit 30 and on to theprocessing unit 35 for analysis using one of the acceptable methods fordetecting likely arrhythmia based upon sensed heart rhythm. The inputdata from the one or more sensors is typically transmitted viawire/circuit board etching. The temporary memory storage unit 30(typically an array) stores the heart beat data over the appropriatetime frame, typically 10-60 seconds, for the processing unit 35 toanalyze the patient's heart rhythm. The processing unit 35 performs thedetection algorithm, shown in either FIG. 2 or FIG. 3 (depending uponthe number of input signals sensed), to determine if there areindications of supraventricular arrhythmia based upon the patient'sbeat-to-beat heart rhythm.

Depending upon the results of the analysis, the processing unit 35 maytransmit output data to the output display unit. In the preferredembodiment of FIG. 5.1, the output display unit is an LCD-type screen 40located atop the central portion of the housing 50, which informs thepatient of the outcome of the test, warning the patient if a potentialarrhythmia is detected. Other possible output display units could bemerely a flashing light to warn the patient, an audible alarm to warnthe patient, or a digitized voice. The CRMD 10 also includes a powersupply source 55 to run the various functions of the device. Althoughany type of power supply source may be used (including a plug for use ina standard wall socket or solar power cells), a battery is used in thepreferred embodiment because it provides dependable power without anyrestrictions to movement. In the preferred embodiment, a standard (overthe counter) type of battery is used. The functional elements of theCRMD 10 are all contained within a housing 50, which protects thefunctional elements from harm and provides for convenient and safehandling of the CRMD 10. The preferred embodiment of the CRMD 10 shownin FIG. 5.1 is approximately seven inches in length, so that it may beconveniently carried by a patient. And in this preferred embodiment, theCRMD 10 employs a housing 50 with a triangular cross-section, in orderto prevent the device from rolling when it is placed upon a flatsurface.

The second variant of the CRMD 10 monitors the patient's heart rhythmcontinuously. For continuous monitoring, it must be worn by the patientover extended periods of time; so for this type of use, the CRMD 10 mustbe configured in a convenient, unobtrusive manner. Although there arenumerous possible ways to configure the CRMD 10 for continuousmonitoring of the patient's heart rhythm, the preferred embodiment ofthis configuration is illustrated in FIG. 6. In this embodiment, thesensors are typically worn as wristbands, one on each of the patient'swrists. The sensor units are approximately the size of a wristwatch, andin the preferred embodiment, the primary sensors are conductiveelectrodes 23 that contact the patient's skin and sense the electricalimpulse generated by the patient's heart. Additional, secondarysupplemental sensors 24 that monitor the patient's mechanical responses,such as a peak blood flow (pulse) pressure sensor or a pulse oximeter,may be include within the wristbands. Furthermore, while the data couldbe transmitted to the processing unit 35 via wires (which would restrictthe patient's freedom of movement), the preferred embodiment utilizes awireless transmitter 60. If the sensors detect mechanical responses,then the sensors will need to be worn on pulse points on the patient'sbody. If the sensors rely exclusively on electrical impulse detection,then location is not as critical. For example, if two conductiveelectrodes 23 are the sole sensor means, then they may be placedanywhere about the patient's body so long as one conductive electrode 23is located on each side of the patient's body.

The processing unit 35, temporary memory storage unit 30, any additionalmemory storage unit, output display unit 40, and power supply source 55may be incorporated within a housing 50 which can be worn by the patienton a belt, in the manner of a Walkman™. The processing unit 35/memoryunit 30 also includes a receiver 65 for the wireless transmission ofdata from the sensor bands in this preferred embodiment. It is alsopossible to incorporate the processor 35, memory storage unit 30,receiver 65, and the output display unit 40 within the housing for oneof the sensor units. In the preferred embodiment for example, shown inFIG. 6, the processor 35, memory storage unit 30, data receiver 65, andoutput display unit 40 are incorporated within one of the sensor unitsfor convenience.

For continuous monitoring, convenience factors are important, so theCRMD 10 in this embodiment is designed to be compact and lightweight, tohave a useable power source life span of at least 24 hours, and to beworn in an unobtrusive manner that does not interfere substantially withdaily activities, etc. Although it is configured differently, since thissecond variant must be worn over some period of time, it functions inthe same essential way to monitor the patient's heart rhythm. If thealgorithm determines that the signal inputs from the sensors indicatethat the patient is experiencing a potential arrhythmia, the processingunit will transmit an output signal to warn the patient.

While the above detailed description has focused on the two primaryversions of the preferred embodiment of the physical form of the CRMD10, the remainder of this detailed description section will focus on themethod employed internally within the CRMD 10 to analyze a patient'sheart rhythm in order to detect likely arrhythmia. The actualdetermination of likely arrhythmia and the decision of whether to warnthe patient is performed by applying a detection algorithm to analyzethe patient's beat-to-beat heartbeat rhythm pattern. In the preferredembodiment, the algorithm is set forth as computer-operated software,with an electronic processing unit analyzing the sensed beat-to-beatheartbeat data of the patient in order to search for signs ofarrhythmia. No specific software or algorithm is required, so long as itperforms the overall function of determining if there is a sustainedpattern of heartbeat intervals outside of the normal physiological rangeof variance, which is indicative of arrhythmia. Indeed, the specificsoftware and/or algorithm may be dependent upon several factors,including the number of sensors available for detecting the patient'sbeat-to-beat heartbeat rhythm, the amount of memory space available forstoring the sensed heartbeat data and/or the intermediate determinationsof whether a particular interval indicates an irregularity, and thechosen method of analysis and level of sensitivity/safety factor desiredin the warning process.

Although there are several variants of the present invention, all ofwhich are generally included within the scope of this description, thepreferred embodiment of the method employed within the CRMD 10 is shownin FIGS. 1, 2, and 3. FIG. 1 is a flowchart which generally illustratesthe initial portion of the preferred embodiment of the arrhythmiadetection algorithm, while FIGS. 2 and 3 are alternative versions of thesecond portion (termed the analysis portion) of the arrhythmia detectionalgorithm. FIG. 2 illustrates the analysis portion of the preferredembodiment of the arrhythmia detection algorithm when only one heartbeat sensor is utilized, whereas FIG. 3 illustrates the analysis portionof the preferred embodiment of the arrhythmia detection algorithm whenthere is a primary sensor and at least one secondary sensor.

Furthermore, as has been briefly explained above, while there areseveral possible methods for analyzing beat-to-beat heartbeat intervalsin order to determine if a patient is likely experiencing an arrhythmiaand should, therefore, seek medical attention, in the preferredembodiment a tri-phase analysis based on statistical analysis of thespecific patient's heart rhythm is utilized. This tri-phase analysis isdemonstrated in both FIGS. 2 and 3, with FIGS. 4.1 and 4.2 furtherdetailing the first and second phases of the analysis process. It isunderstood, however, that this is in no way intended to limit the scopeof this invention. Rather, the analysis portion of the algorithm may beany of various possible alternative methods available, depending uponthe specific needs of the design. By way of example, either of the PhaseOne or Phase Two analysis could be used alone in place of the tri-phaseanalysis of the preferred embodiment, although accuracy may be reduced.Similarly, a pre-constructed table based upon data from a generalpopulation study could be used to determine whether a patient'sbeat-to-beat heartbeat intervals were outside of the normal variancerange. A person skilled in the art field will appreciate that these andother analysis techniques would apply as alternatives to the tri-phaseanalysis set forth in the preferred embodiment, and this invention isintended to include all such variations. The preferred embodimentutilizes the tri-phase statistical analysis because it is patientspecific and provides additional accuracy, reducing the likelihood offalse positives.

Turning now to the drawings of the preferred embodiment in more detail,FIG. 1 illustrates the initial portion of the arrhythmia detectionalgorithm. In the preferred embodiment, the CRMD 10 is configured sothat it is activated automatically when the patient grips bothconductive electrodes 23. Alternative means for activating the CRMD 10,such as an on/off power switch or button or even operating the device ona solar cell which automatically activates when there is sufficientlight available to power the device, do exist and could also be used.

Regardless, the process begins by sensing the patient's beat-to-beatheartbeat intervals. Any of several types of reliable heart beat sensorscould be used, including both electrical sensors andmechanical/physiological sensors, such as pulse oximeters, thermisters,optical electrodes, peak blood flow (pulse) and pressure sensors. In thepreferred embodiment, the primary sensing means utilizes conductiveelectrodes 23 to detect the electrical impulses generated by thepatient's heart in order to measure the patient's beat-to-beat heartbeatintervals. Optionally, depending upon whether the analysis portion ofthe arrhythmia detection algorithm (either FIG. 2 or FIG. 3) utilizesadditional sensor input for additional accuracy and/or to provideback-up sensing capabilities, one or more secondary sensing means mayalso be utilized to detect the patient's beat-to-beat heartbeat rhythm.If additional, secondary sensors are employed, then another optionalfeature may be used to determine which of the several available sensorinput signals should be treated as the primary sensor (rather thanmerely designating the primary sensor based upon general performancestandards for the various sensors). If this optional step is included,then the sensor with the highest available stable input signal amplitudewould be selected as the primary sensor, and the remaining sensors wouldbe designated as secondary sensors.

Another optional feature (not shown in FIG. 1) would be to verify thatboth the electrical and mechanical/physiological signals roughlycorrespond. In this way, the CRMD 10 would preliminarily screen foroutside sources of error. For example, if there is an electrical signalbut no mechanical signal, this would tend to indicate a problem whichmay make the analysis algorithm operate inaccurately. It could, forinstance, indicate that the patient has not properly gripped the CRMD10, so that the mechanical sensors are unable to detect their heartbeat.Regardless, if there are disparate sensor readings from the varioussensors, then an error message should be transmitted. The patient'sheart rate is also optionally examined to ensure that it is withinnormal resting range, since the detection algorithm is formulated basedupon the assumption of a patient at rest, and the accuracy of theanalysis may be affected if the patient's heart rhythm is undergoingstress. Although there are several means for defining the normal restingheart rate range, which a person skilled in the art will be familiarwith, in the preferred embodiment the normal resting range is defined asapproximately between 35 to 220 beats per minute. If the patient's heartrate falls outside of the normal resting range, then an error message isdisplayed and additional heartbeats will continue to be sensed untileither the patient's heart rate enters the normal resting range or atime limit (of two minutes in the preferred embodiment) expires.

Once the available sensors of the CRMD 10 are sensing the patient'sbeat-to-beat heartbeat intervals, noise and bandwidth filtering aregenerally applied in order to screen out any background electrical noiseand interference. Typically, the filter is set to screen out any signalsbeyond the level of magnitude of the physiological range of the humanheart. This reduces sources of external error, providing for a moreaccurate arrhythmia detection process. Then, the analog signals from thesensors (representing the patient's beat-to-beat heartbeat rhythm) areconverted to digital signals, using a digitizer, for more efficientprocessing (utilizing a computerized processing means). The digital datafrom these signals are stored, so that the analysis algorithm (FIG. 2 orFIG. 3) can process the data and analyze the patient's beat-to-beatheartbeat intervals for indications of arrhythmia. In the preferredembodiment, the digital data for the patient's sensed beat-to-beatheartbeat interval rhythm is stored in a separate computerized array foreach sensor. The data will continuously be sensed and stored in the oneor more arrays so long as the patient is using the CRMD 10, but theanalysis algorithm will take place using discreet sets of data withinthe arrays. The analysis algorithm will begin once a sufficient numberof data points representing beat-to-beat heartbeat intervals for thepatient have been collected in order to complete a set for analysis,although data will continue to be sensed and stored while the analysisalgorithm is operating in case additional information is needed forlater processing in Phase Three of the tri-phase analysis utilized bythe preferred embodiment of the CRMD 10.

Typically, the minimum number of beat-to-beat heartbeat intervalsrequired for a set for proper analysis is eight, since this provides apattern with a statistically significant number of heartbeat intervalsfor an adequate level of accuracy. While slightly smaller data sets maybe possible, they raise the risk of increasing errors beyond anacceptable level, resulting in an inordinate amount of false positives.Obviously, the larger the data set used for analysis, the more accuratethe algorithm outcome is likely to be, as larger data sets reduce theeffect of the occasional aberrant heartbeat and provide a broaderpicture of the patient's heart rhythm. In the preferred embodiment, theset has been enlarged to include thirty-two data points for increasedaccuracy.

Once the array has stored a sufficient number of heart beat data pointsfor a set, the CRMD 10 begins the analysis portion of the arrhythmiadetection algorithm, as shown in FIGS. 2 and 3 for the preferredembodiment. Both FIGS. 2 and 3 utilize generally the same basic analysismethod, termed tri-phase analysis, to determine if there is a likelyarrhythmia. FIG. 2 is simpler in that it includes only a single sensorand so the analysis only operates on a single set of data stored in asingle array. FIG. 3 is quite similar to FIG. 2 in overall purpose, butinvolves some added complexity since it utilizes at least one secondarysensor data set in addition to the data set from the primary sensor.

The tri-phase analysis utilized in the preferred embodiment is only oneof several methods for analyzing the patient's beat-to-beat heartbeatdata for indications of arrhythmia. There are two primary types ofalgorithms which may be used. The more basic analysis technique would beto compare the patient's beat-to-beat heartbeat interval variance to apre-constructed table based upon a general population study, in order todetermine if each interval variance falls outside of the normal varianceof time between heartbeats for a healthy person. So, for instance, theR-R interval of a particular heart beat would be compared to the averageR-R interval between heartbeats for the patient over the data set, andthe algorithm would keep track of the number of times that the varianceexceeded the normal range for such variance as determined from a generalpopulation study, typically approximately 125 milliseconds based uponsome current studies. Any variance outside of the normal range ofvariance would be a potential irregular heartbeat, and if a sufficientnumber of irregular heart beats were detected over a span of time, thenthat would indicate a potential arrhythmia and the CRMD 10 would warnthe patient.

The second analysis technique uses individualized statistical analysisof the patient's own normalized heart rhythm, rather than somecomparison to a general population study, in order to classifyheartbeats as irregular, and then identifies heart rhythms with asufficient number of irregular heartbeats as unstable and possiblyindicative of arrhythmia. This is the preferred method of analysis,since it is patient-specific. In the preferred embodiment, the patient'sheartbeat data is normalized and then standard normal distributionanalysis is employed to determine if a particular heartbeat intervalindicates an unstable heart rhythm which, when viewed as part of apattern, could indicate arrhythmia. This is the method employed in thepreferred embodiment. Actually, the preferred embodiment takes thistechnique a step further, utilizing multiple methods for normalizing thepatient's heart rhythm in order to increase the accuracy of theanalysis. This is certainly not required, and either the Phase One orPhase Two types of analysis could be utilized independently, along withother variations of normalization techniques familiar to persons skilledin the art field; but the tri-phase analysis algorithm provides anadditional check, reducing the chances of incorrect outcomes.

Turning first to FIG. 2 and the simpler of the analysis algorithms, thebasic structure of the algorithm involves three separate phases ofanalysis when there is only a single data set from a single heartbeatsensor. First, the preferred embodiment of the CRMD 10 runs the PhaseOne analysis shown in FIG. 4.1 on the patient's beat-to-beat heartbeatinterval data set. Each sensed beat-to-beat heartbeat interval stored inthe array is divided by the following sensed beat-to-beat heartbeatinterval stored in the array (for example, the interval stored in thefirst array slot is divided by the interval stored in the second arrayslot), in order to normalize the data. Each normalized heartbeatinterval is then considered using normal distribution analysisprinciples in order to determine if it should be classified asindicative of a stable or unstable trend. Although various criteriacould be used to determine stability, depending for example on the levelof accuracy and sensitivity desired for the CRMD 10, in the preferredembodiment, any normalized value which falls outside of the range from0.95 to 1.05 would be deemed unstable. The overall outcome of Phase Oneis determined by comparing the number of “Stable” results to the numberof “Unstable” results, with the Phase One overall outcome being set as“stable” if the number of stable results is greater than the number ofunstable results and the Phase One overall outcome being set as“unstable” if the number of unstable results is greater than the numberof stable results (and the data set should ideally be set to result inan odd number of results so that there is a definitive determinationfrom the Phase One analysis).

Then, the preferred embodiment of the CRMD 10 performs the Phase Twoanalysis shown in FIG. 4.2 on the patient's beat-to-beat heartbeatinterval data set. The average beat-to-beat heartbeat interval for thedata set is calculated first, since this is the normalizing factor forPhase Two. Each sensed beat-to-beat heartbeat interval stored in thearray is divided by the average beat-to-beat heartbeat interval for thedata set, in order to normalize the data. Each normalized heartbeatinterval is then considered using normal distribution analysisprinciples in order to determine if it should be classified asindicative of a stable or unstable trend. Although various criteriacould be used to determine stability, depending for example on the levelof accuracy and sensitivity desired for the CRMD 10, in the preferredembodiment, any normalized value which falls outside of the range from0.95 to 1.05 would be deemed unstable. The overall outcome of Phase Twois determined by comparing the number of “Stable” results to the numberof “Unstable” results, with the Phase Two overall outcome being set as“stable” if the number of stable results is greater than the number ofunstable results and the Phase Two overall outcome being set as“unstable” if the number of unstable results is greater than the numberof stable results (and the data set should ideally be set to result inan odd number of results so that there is a definitive determinationfrom the Phase Two analysis, so the preferred embodiment does notexamine the final stored heart beat interval).

Phase Three of the preferred embodiment, as shown on FIG. 2, comparesthe overall outcome for Phase One and Phase Two. If the Phase Oneoutcome matches the Phase Two outcome, then the final overall result ofthe tri-phase analysis is this uniform verdict (i.e. if both Phase Oneand Phase Two indicate that the patient's heart rhythm is “stable,” thenthe final overall result of the tri-phase analysis is “stable;” if bothsay “unstable,” then the final overall result of the tri-phase analysisis “unstable”). If the final overall result of the tri-phase analysis is“stable,” then the CRMD 10 outputs an all clear signal to the patientusing the available output display device 40. If the final overallresult of the tri-phase analysis is “unstable,” then the CRMD 10 outputsa warning to the patient using the available output display device 40,indicating possible arrhythmia and advising the patient to seek medicalattention in the preferred embodiment.

If, on the other hand, the Phase One outcome does not match the PhaseTwo outcome, then Phase Three will take the next data set (of 32heartbeat intervals in the preferred embodiment) of the patient'sbeat-to-beat heart beat intervals stored in the array and re-run boththe Phase One and Phase Two Analyses on the new data before againchecking to see if the Phase One outcome matches the Phase Two outcome.This iterative approach will occur until either the Phase One outcomematches the Phase Two outcome, or until Phase Three has failed toproduce a matching result after a pre-set number of iterations. In thepreferred embodiment, if there is no match by the third iteration, thenthe CRMD 10 will output a signal indicating inconclusive results andwarning of possible arrhythmia to the output display device 40. Anexample of the tri-phase analysis can be seen in FIGS. 4.3 and 4.4. FIG.4.3 illustrates, using a data set of eight beat-to-beat heartbeatintervals as an example, the kind of analysis which would occur in thetri-phase analysis algorithm of FIG. 2 when both the Phase One outcomeand the Phase Two outcome match (here providing a “Stable” result, forexample). FIG. 4.4 illustrates, using a data set of eight beat-to-beatheartbeat intervals as an example, the kind of analysis which wouldoccur in the tri-phase analysis algorithm when the Phase One outcome andthe Phase Two outcome do not match, such that Phase Three must re-runthe analysis on the next set of data in order to iteratively seekagreement.

It should be understood that either Phase One or Phase Two could be usedalone, rather than as part of this tri-phase analysis of a patient'sheart rhythm. Doing so would tend to reduce the accuracy of theanalysis, however, and so to compensate it may be wise to increase thesize of the data sets, to run multiple separate data sets using thespecific analysis technique chosen, or to increase the sensitivity ofthe criteria for determining if a normalized heartbeat is stable orunstable. Furthermore, the pre-constructed chart technique discussedabove could also be used alone or in tandem with either the Phase One orPhase Two technique for the analysis portion of the arrhythmia detectionalgorithm of the CRMD 10.

FIG. 3 illustrates a similar preferred embodiment of the analysisportion of the arrhythmia detection algorithm when multiple sensors areavailable, providing multiple simultaneous beat-to-beat heartbeatinterval readings of the patient's heart rhythm. FIG. 3 assumes aprimary sensor and a secondary sensor, such that there is a primarysensor array which stores the data for the beat-to-beat heartbeatintervals from the primary sensor and a secondary sensor array whichstores the data for the beat-to-beat heartbeat intervals from thesecondary sensor. In operation, the tri-phase analysis shown in FIG. 3is very similar to that shown in FIG. 2, except that it is applied toboth the data of the primary sensor array and the data of the secondarysensor array in order to provide additional accuracy.

In Phase Three, the final overall result is set to whicheverclassification (either stable or unstable) is uniformly held by thePhase One and Phase Two outcomes for both the primary and secondary datasets, unless there is not such uniform agreement, in which case thealgorithm iteratively re-runs Phase One and Phase Two again for bothdata sets until either uniform agreement is found or until the pre-setcut-off limit activates and causes the CRMD 10 to output a warning.Given the fact that each additional sensor increases the chances fordiscrepancies, it may be advisable to increase the number of iterationsPhase Three will perform when the final outcomes of Phases One and PhaseTwo do not match in order to fine tune the tolerance/sensitivity of theCRMD 10. Other possibilities for adjusting the sensitivity of the CRMD10 might include processing larger data sets for increased accuracyand/or adjusting the selection criteria for determining whether aparticular normalized heartbeat is indicative of instability.

A person skilled in the art field will understand that the heart rhythmanalysis methods detailed above are merely illustrative and are notintended to limit the scope of this invention. These and other analysismethods will be apparent to those skilled in the art field, and any oneor combination of accepted analysis techniques may be utilized withinthe CRMD 10 in order to detect potential arrhythmia and to warn thepatient to seek medical attention. Furthermore, the specific details ofthe analysis techniques may be set depending upon the specificcircumstances for which the CRMD 10 will be used, with such designfactors as the number of different sensors used, the number of differentanalysis techniques used, the number of beat-to-beat heartbeat datapoints in a data set for analysis, the specific criteria for definingwhether a particular sensed heart beat is indicative of irregularity,and the number of irregularities in a given data set or over a givenperiod of time which is indicative of an unstable heart rhythm which mayindicate a possible arrhythmia set according to particular circumstances(such as the level of accuracy desired, the sensitivity desired, theaverage amount of time that the analysis should take, the acceptablenumber of false positive results, etc.). The preferred embodiment ismerely illustrative of one method for analyzing a patient's heart rhythmwithin the scope of the CRMD 10 device.

1. A method for detecting potential arrhythmia comprising the steps of:sensing beat-to-beat heartbeat intervals; storing beat-to-beat heartbeatinterval data; analyzing beat-to-beat heartbeat interval data to detectsigns of potential arrhythmia by noting instances when the variance forindividual sensed beat-to-beat heartbeat interval data falls outside ofthe normal range; and outputting a warning when said number of instancesoutside of the normal range indicates a pattern of irregularity.
 2. Amethod as in claim 1 wherein the arrhythmia detection analysis utilizesstatistical normalization techniques and normal distribution analysis todetermine if a particular beat-to-beat heartbeat interval variance fallsoutside of the normal range.
 3. A method as in claim 1 wherein thearrhythmia detection analysis calculates the variance for each sensedbeat-to-beat heartbeat interval based on the difference with the averageheartbeat interval, and utilizes the results from general populationstudies to determine the normal range of variance.